In conventional molding practice, alignment of the flask to the pattern, and flask-to-flask, is achieved by means of pins and bushings. More specifically, in most instances pins are provided on the pattern which can engage bushings on the drag flask at the mold making station. These same bushings are then used to engage pins on the cope flask at the closing station, where the drag and cope flasks are assembled. The above achieves the alignment requirement for patterns to flask, and flask-to-flask.
One of the problems frequently encountered in conventional molding systems is a casting flaw attributable to parting line shift. One of the principle causes of this casting defect is the general problem associated with the clearances required between pins and bushings, and the increase in those clearances caused by wear on these critical alignment parts. Typically, clearances on the order of ten thousandths of an inch are required between pattern pins and their mating bushings on the drag in order to prevent binding action as the flask is lowered onto the pattern or later on when it is drawn. Similar clearances are required in making the cope, and the same clearances are also then encountered in the match between the cope and the drag.
In addition to these initial clearances, it is not uncommon to see wear on pins and/or bushings which can often accumulate to an additional ten thousandths of an inch. It can therefore be seen that with worn pins and bushings, a drag might be shifted relative to its pattern as much as fifteen thousandths from a theoretical true desired position. Similarly, a cope can be shifted to a corresponding amount, but in the opposite direction. When the cope and drag are eventually closed, an error of as much as forty-five thousandths can occur between the cope and drag parting surfaces from these variables alone.